This invention relates to a thermostat adapted to open a circuit in response to temperature change and relates more particularly to a thermostat having heating means for keeping the circuit open until the thermostat is manually reset.
A known thermostat comprises a bimetal switch element reacting upon temperatures, which on one of its ends is fastened on a frame and on the other end is provided with an electric contact adapted to engage a fixed contact provided on the frame, said frame comprising two mutually parallel sheet-like metallic contact carriers forming a base unit, through which two parallel pins are extending for connection of both contact carriers and on which pins the contact carriers, with a tight fit, are slidable to and fro relative to one another to be held permanent mutually parallel during mounting, which thermostat may be provided with an external heating means and/or an internal heating means, and a selected heat capacity, as well as with various embodiments of connecting terminals for the contact carriers.
Such a thermostat has been described in U.S. Pat. No. 4,528,540.
A number of embodiments of the known thermostat are shown in the noted patent for influencing the current sensibility and/or the times of switching the thermostat on and off. The patent also shows various connection and mounting possibilities.
For instance various embodiments of connecting terminals for the contact carriers are shown: an external heating element that is fed by the current to be switched is shown, possibly being arranged in or on means of a selected heat capacity; and the contact carriers, one or both of which being made in a resistance material, are shown being possibly combined with a special configuration of said carriers and/or an additional means of selected heat capacity.
Furthermore the possibility is indicated in the patent of adding to these embodiments a heat source preferably with PTC characteristics, connected in parallel, for keeping the thermostat open after being opened until the main current is interrupted, so that a so-called manual reset function is obtained.
A drawback of this structure is that an additional component (requiring additional space) and contact making means has to be added, whilst special care should be bestowed on stability, for instance in the event of vibrations. It is a further drawback that in the automatic reset function, that is without an additional heat source connected in parallel, the switch-back times highly depend upon the ambient temperature, and the range of switch-back times is restricted i.e. by the difference between the switch-back temperature and the ambient temperature.